The invention relates to a method for generating a foam from a liquid phase and a gas phase, a method for placing a foam in circulation in an installation, and a method for cleaning an installation by placing a foam in circulation.
The invention also relates to a system for generating a foam and to a system for generating and placing a foam in circulation in an installation.
The method of the invention may be of use for example in a method for cleaning and/or decontaminating an installation by means of a foam. Liquid phase cleaning and/or decontaminating methods for a large volume installation, having for example a complex inner geometry, generate considerable volumes of waste. The use of a foam, containing one or more cleaning and/or decontaminating agents brings a significant reduction in the volumes of waste generated. The cleaning and or decontamination of an installation is made by injection of the foam inside the installation to be cleaned and/or decontaminated, and at times by placing the foam in circulation within these installations.
The method of the invention is particularly advantageous for cleaning and/or decontaminating installations operating under low pressure such as a pneumatic carrier network for samples intended for analysis, a ventilation circuit or pipes which have undergone radioactive contamination.
Foam generation is generally conducted by mechanical shaking of a liquid, by sudden depressurising of a gas solubilized in a liquid, or by injection of gas and liquid under pressure at the inlet of a static porous medium.
For example patent application EP-A-0 526 305 describes firstly a method for preparing a foam consisting of causing a gas under pressure to pass through a sintered plate in the presence of a solution, the solution and the gas being suitable to form a foam.
The document previously cited also describes a method for cleaning an installation in which the foam is propelled into the installation by the pressure of the gas used to generate the foam. The flow rate of the gas and liquid are set so as to generate foam on entering the installation, irrespective of the characteristics of said installation to be cleaned. The method of preparing a foam and for cleaning an installation described in this document are not appropriate for cleaning sensitive installations, in particular installations for which a pressure greater than atmospheric pressure is prohibited.
It is therefore necessary to put forward a system for generating and placing a foam in circulation which operates at pressures of atmospheric pressure or less.
The purpose of the present invention is to provide a method for generating a foam from a liquid phase and from a gas phase with which homogeneous foam can be generated having few or no air pockets.
The method of the invention is characterised in that it comprises a step consisting of generating the foam by aspiration of the liquid phase and the gas phase through a porous lining.
The principle of the method of the invention consists of no longer injecting liquid and gas phases under pressure through the porous lining, but of draining them through the pores or interstices of the lining by setting up constant low pressure downstream from this lining.
The gas phase and the liquid phase are aspirated simultaneously through the lining under the effect of low pressure. The porous lining therefore acts as a contacter between the gas phase and the liquid phase.
The gas phase-liquid phase mixture is made in the porous lining in which interfaces are created and therefore foam is created. The energy required for this mixture and the creation of interfaces is provided by the flow of the liquid and gas phases in the lining under the effect of low pressure.
In order to obtain a foam of constant quality when it leaves the lining, various variables need to be controlled which come into play during the generation method described above. These variables are the chemical composition of the liquid phase, also called foaming solution, the flow rate of the liquid phase arriving in contact with the porous lining, the flow rate of the gas phase drawn by aspiration, the geometry of the porous lining placed in a chamber, and the geometry of said chamber.
The chemical composition of the foaming solution is chosen in relation to the intended use of the generated foam. The foam may, for example, be a cleaning and/or decontaminating foam for an installation, and/or a scouring foam, a rinsing foam, a foam intended to apply a film having surfactant or bactericidal properties for example.
The quality of the foam may be determined for example by a lifetime, a moisture content, or its expansion. The lifetime of a foam may be defined as the time required for total conversion of a given volume of foam into liquid and gas. The moisture content of a foam may be defined as the ratio between the liquid phase volume and foam volume. Expansion F of a foam is defined, under normal temperature and pressure conditions by the following ratio (1):   F  =                              V          gas                +                  V          liquid                            V        liquid              =                  V        foam                    V        liquid            
in which:
F=expansion in units of expansion
Vgas=the volume of the gas phase in the foam
Vliquid=the volume of the liquid phase in the foam
Vfoam=the volume of foam.
A foam having constant quality will have constant expansion. Generally, the foams prepared with the methods of the prior art have an expansion in the order of 10 to 15. Expansion also provides a magnitude of the amount of decrease in the volume of generated liquid waste for example, when the foam is used to clean an installation.
Expansion also makes it possible to assess the quantity of air pockets present in the foam, and therefore to assess the quality of this foam.
If the foam is intended to carry out cleaning and/or decontamination and/or scouring, according to the method of the invention, the liquid phase may also comprise at least one foaming surfactant conventionally used to generate a foam, at least one foam stabilising or destabilising agent with which it is possible to modify the lifetime of the foam or its moisture content, and/or at least one cleaning agent and/or at least one decontaminating agent and/or at least one scouring agent for an installation.
If the foam is intended to carry out rinsing of an installation, the liquid phase may be an aqueous solution of at least one surfactant agent and of at least one foam destabilising agent.
In a foam composition that can be used to implement the method of the invention, the constituents of the liquid phase, in particular the foam destabilising agent, and their quantity are chosen such as to obtain a foam lifetime of 15 to 30 minutes and a moisture content of 2 to 20%.
Examples of appropriate liquid phases to implement the method of the invention are described in EP-A-0 526 305.
The destabilising agent may be an organic compound which destabilises the foam by acting on dynamic surface tension, for example an alcohol preferably having a boiling point slightly higher than that of water, for example a boiling point of 110xc2x0 C. to 130xc2x0 C. Preferably a C5 to C6 secondary alcohol is used, such as pentanol-2.
Generally the quantity of destabilisation agents represents from 0.2 to 1% by weight of the liquid phase.
In the liquid phase of the foam, the decontamination reagent may be made up of reagents routinely used in wet process decontamination methods. If the objects to be decontaminated are in metal, particular use is made of reagents made up of inorganic or organic acids or bases. As an example of acid reagents, mention may be made of hydrochloric acid, nitric acid, sulphuric acid and phosphoric acid which may be used alone or in combination. It is also possible to use organic reagents such as citric or oxalic acids.
As an example of basic reagents NAOH, KOH and their mixtures may be cited, to which oxidants for example may be added, such as H2O2 or the permanganate ion.
In the case of acid reagents, their concentration in the liquid phase may range for example up to 10 mol.1xe2x88x921; for base reagents, their concentration may for example reach 5 mol.1xe2x88x921.
If an acid reagent is used made up of H2SO4 at a concentration of more than 3 mol.1xe2x88x921, a viscosing compound is preferably added to the liquid phase such as polyethylene glycol, for example polyethylene glycol with an average molecular weight of 6000. Sulphuric acid accelerates a phenomenon of direct sedimentation of the liquid phase through the interface separating the gas bubbles from the foam, but this may be slowed down by means of this viscosing compound.
Generally the concentration of viscosing compound in the liquid phase does not exceed 1% by weight.
The liquid phase of the foam also contains at least one surfactant agent to promote foam formation, preferably two surfactant agents are used, respectively a betaine, in particular a sulfobetaine, and an oligosaccharide alkyl ether. The association of these two surfactant agents is of interest as it remains surface active irrespective of pH, and is therefore suitable both for a neutral medium, for example for the rinsing of an installation, and for an acid or basic medium, that is to say with acid or basic. decontamination reagents.
Generally, the concentration of betaine is 0.2 to 0.5% by weight and the concentration of the oligosaccharide alkyl ether is between 0.3 and 1% by weight.
It is possible for example to use a sulfobetaine such as that sold by SEPPIC under the trade name AMONYL (registered trade mark).
As an example of oligosaccharide alkyl ether which may be used as second surfactant, mention may be made of that sold by SEPPIC under the trade name ORAMIX CG110 (registered trade mark,) and that marketed by ROHM and HASS under the trade name TRITON CG60 (registered trade mark).
As seen above, the contents of surfactant agents, and/or of stabilising or destabilising agents are chosen in relation to the lifetime of the foam it is wished to obtain. If the foam is intended for cleaning and/or for decontaminating an installation, the contents of decontamination and/or cleaning reagents are chosen in relation to the type of items to be decontaminated and/or cleaned and to the type or extent of required decontamination and/cleaning.
As an example, the liquid phase of a foam, for example a rinsing foam, which can be used according to the method of the invention, may be made up of an aqueous solution containing:
from 0.2 to 0.5% by weight of betaine,
from 0.3 to 1% by weight of an oligosaccharide alkyl ether, and optionally
from 0.2 to 1% by weight of a destabilising agent.
In a further example, the liquid phase of a foam, for example a decontamination foam, which may be used according to the invention, may be made up of an aqueous solution containing:
3 to 6 mol.1xe2x88x921 sulphuric acid,
0.1 to 1% by weight of a viscosing compound,
0.2 to 0.5% by weight of betaine,
0.3 to 1% by weight of an oligosaccharide alkyl ether, and optionally,
0.2 to 1% by weight of a destabilising agent.
In another example, the liquid phase of a foam, for example a scouring foam, which may be used according to the invention, may be made up of an aqueous solution containing:
3 to 5 mol.1xe2x88x921 NaOH,
0.1 to 1% by weight of a viscosing compound,
0.2 to 0.5% by weight of betaine,
0.3 to 1% by weight of an oligosaccharide alkyl ether, and optionally
0.2 to 1% by weight of a destabilising agent.
Another variable which contributes to the quality of the foam generated with the method of the invention is the flow rate of the liquid phase arriving in contact with the porous lining. This flow rate may be set by means of a measuring pump. Depending upon the quality of the required foam, the flow rate of the liquid phase is adjusted in relation to the flow rate of the gas phase and to the aspiration of the liquid and gas phases through the porous lining. The flow rate of the liquid phase must also be adjusted in relation to the porous lining, in particular in relation to the pore size of this lining.
Foam quality may also depend upon the manner in which the liquid arrives in contact with the porous lining; by promoting the formation of a coarse foam as soon as contact is made with the porous lining, the quality of the generated foam can be increased. Therefore the mode in which the liquid is sprayed onto the surface of the lining has an influence which may also lead to its greater or smaller homogeneous distribution. The arrival of the liquid phase in contact with the lining may be made for example by means of a spray nozzle, or even by inserting a grid between the arrival of the liquid phase in the chamber and the porous lining, that is to say above the porous lining.
Another variable which acts on the quality of generated foam is the low pressure prevailing downstream from the porous lining, this low pressure causing aspiration of the liquid and gas phases through the porous lining. Also the flow rate of the generated foam is related to this low pressure downstream from the porous lining. In practice, the low pressure chosen must take into account the loss of pressure in the porous lining. On this account, the flow rate of the foam can be controlled when it leaves the porous lining by means of a flow meter, and the value of this flow can be adjusted by means of a low pressure adjustment system.
A further variable acting on the quality of the foam generated by the method of the invention is the type of lining used for this generation. This lining may be any medium offering a throughway permitting flow of the liquid phase and gas phase through the porous lining in order to assure their mixing. The pore openings of the porous lining may preferably be uniformly distributed within the lining volume, these openings preferably being of small size, for example from 100 xcexcm to a few mm, in order to promote the mixing of the gas phase and the liquid phase and to avoid the onset of air pockets in the foam. However, pores that are too fine may generate considerable pressure losses.
By way of example, the porous lining may, at choice, be either a stack of metal grids, a knitted synthetic fabric of FORAFLON type (registered trade mark), sand, diatoma or perlites, solid gauged beads, or any other material having adequate interstices for foam generation.
Preferably, according to the method of the invention, gauged beads are used, for example gauged glass beads. The value of the pressure loss in the porous medium can then be controlled in precise, reproducible manner by the thickness of the bed of beads and bead diameter. For a bed of gauged glass beads, it is possible firstly to use as basis two conventional ratios valid for incompressible, homogeneous, Newtonian fluids.
Firstly the DARCY ratio which relates a flow rate U of a liquid phase, or velocity of a liquid phase in m/s, a viscosity xcexc of the liquid phase in Pa.s, the thickness z of the porous lining crossed by the liquid phase and the gas phase in meters, and the difference in pressure xcex94P in Pascal units between the pressure P1 upstream from the porous lining, and the pressure P2 downstream from the porous lining, is written U=B.xcex94P/xcexc.z in which xcex94P=P1xe2x88x92P2 and P1 greater than P2.
Factor B expressed in m2 is called permeability. This factor is characteristic of the porous medium and is related to its geometry.
Secondly, the KOZENY-CARMAN model permits calculation of the permeability B of a porous medium made up of gauged spheres. The mathematical expression of this model will not be detailed herein. It will be simply be recalled that for an incompressible, Newtonian fluid, permeability is inversely proportional to the square diameter of the spheres forming the bed.
For example, for a liquid phase flow rate of up to 100 l/h, preferably from 5 to 50 l/h, passing through a porous lining with a thickness of 0.08 m and formed of glass beads having a diameter of 1.6 mm, the low pressure set up downstream from the porous lining may be between 5xc3x97103 and 80xc3x97103 Pa, preferably between 30xc3x97103 and 60xc3x97103 Pa.
A further variable acting on the quality of the generated foam is the shape of the chamber in which the porous lining is placed. It can for example be envisioned to increase the surface area of the free section of this chamber having a constant lining thickness, a constant liquid phase flow rate and a constant low pressure, in order to enrich the gas, mixture. The chamber may be covered with a lid having at least one opening to permit inflow of the gas chosen to produce the foam, or it may be uncovered if the gas used to generate the foam is ambient air. The flow rate of the foam leaving the porous lining will therefore also be related to the geometry of the chamber.
With the method of the invention, it is possible to generate foams having an expansion of 5 to 40.
According to the invention, the gas phase used to implement the method if the invention may be air, nitrogen, oxygen, a neutral gas such as argon or helium which may be used alone or in combination.
The invention also relates to a method for placing a foam in circulation in an installation, comprising a step consisting of generating a foam by aspiration of an appropriate liquid phase and an appropriate gas phase through a porous lining at a first end of the installation, such that the generated foam is inserted into said installation and travels through it as far as a second end of the installation, aspiration being conducted by setting up a low pressure in said installation at and after said second end.
According to this method, the low pressure set up in the installation at and after said second end initiates aspiration of the liquid and gas phases through the porous lining and subsequently the placing in circulation of the foam inside the installation.
According to the invention, this method of placing a foam in circulation in an installation may be applied to a method for cleaning an installation with a cleaning foam. The liquid phase then comprises one or more cleaning agents.
If cleaning also comprises scouring, the liquid phase may also contain a scouring agent.
According to the invention, the cleaning foam may also be a decontaminating foam, and in this case comprises one or more decontaminating agents.
These decontaminating agents may, for example, be radioactive or bactericidal decontamination agents depending upon the installation to be cleaned.
The cleaning and decontaminating agents are those previously described.
According to a first embodiment of the method of the invention, the foam may be received in a collecting tank after the second end of the chamber and be naturally, chemically and/or mechanically destabilised. Natural destabilisation is made with the use of a foam having a limited lifetime, chemical destabilisation is achieved by adding to the foam, in this collecting tank, one of the destabilisation agents previously cited, and mechanical destabilisation may be made by means of an ultrasound generator for example, a centrifuging machine or a blade turbine.
According to a second embodiment of the method for placing a foam in circulation in an installation, the method may, in addition, comprise the steps consisting of collecting the foam after the second end of the installation, destabilising the collected foam such as to obtain a liquid, and of using at least part of said liquid as liquid phase to generate the foam placed in circulation in said installation. This embodiment may also be called recycling mode.
According to one preferred variant of the second embodiment of the invention just described, the liquid may be purified before being used as liquid phase to generate foam. The purpose of this purification is for example, in respect of installation cleaning and/or decontaminating methods, to remove the waste carried by the circulation of the foam in the installation. This purification may be made by means of adequate filters for example.
The invention also relates to a foam generating system to apply the method of the invention. This system comprises:
a chamber comprising at least one inlet opening and one outlet opening,
a porous lining placed between the inlet and outlet openings of the chamber,
means for inserting in said chamber a liquid phase and a gas phase through said, at least one, inlet opening,
means for aspirating said liquid phase and said gas phase through the porous lining, the generated foam being evacuated from said chamber by said aspiration means through said, at least one, outlet opening.
The chamber may be of any shape, of round shape for example, and made of a material which may be chosen in relation to the porous lining, to the liquid phase, and gas phase used, and in relation to the low pressure applied to generate the foam. This chamber is preferably impervious.
If the gas used is ambient air, the chamber may be uncovered.
The porous lining which may possibly be used is described above.
The insertion means used to add a liquid phase to said chamber through at least one inlet opening may, for example, comprise a measuring pump permitting entry of the liquid phase into the chamber, this pump possibly being provided with means to measure liquid phase flow rate, for example a flow meter. This pump may be connected to a preparation and storage tank for the liquid phase.
In order to distribute the liquid phase in homogeneous manner over the porous lining, a spray nozzle or a grid may be used, preferably a spray nozzle. This nozzle or this grid, by assuring proper distribution of the liquid, is able to promote the creation of a coarse foam above the porous lining as soon as the liquid phase enters this lining, thereby increasing the quality of the generated foam.
The means permitting entry of the gas phase into said chamber may comprise adjustment means to adjust the entry pressure of the gas into said chamber and optionally a reservoir for said gas.
If the gas phase is made up of ambient air, aspiration of the liquid and gas phases through the porous lining causes aspiration of ambient air, in which case, upstream from the porous lining, at least one inlet may be provided on the chamber for ambient atmospheric air, optionally equipped with a flow meter.
The aspiration means for said liquid phase and said gas phase through the porous lining, or means for creating a low pressure, may for example be a vacuum pump optionally fitted with a condensate trap, this pump being able to conduct evacuation of the generated foam from the chamber.
The system may be equipped with a valve or solenoid valve used to set and adjust the low pressure downstream from the lining in the chamber. The system of the invention may also be equipped with measuring means to measure the low pressure in said chamber.
The invention also related to a system for placing a foam in circulation in an installation, the installation comprising a first end and a second end, the first and second ends delimiting at least part of the installation in which the foam is to be placed in circulation, this system comprising:
a foam generation system such as described previously, and
sealed connection means between said, at least, one outlet opening of the chamber and the first end of the installation,
the aspiration means for said liquid phase and said gas phase through the porous lining being positioned at the second end of the installation such as to set up a low pressure in said part of the installation in which the foam is to be placed in circulation.
The system for placing a foam in circulation in an installation is particularly advantageous for cleaning and/or decontaminating said installation.
The chamber, the lining, the insertion means for adding the liquid phase to said chamber, and the insertion means for adding the gas phase to said chamber may be those previously described. The sealed connection means may for example be seals designed such as to withstand the chemical composition of the generated foam, and to withstand the low pressure required to generate the foam by aspiration of the liquid and gas phases through the porous lining.
The aspiration means to aspirate the liquid and gas phases through the porous lining and set up a low pressure in said part of the installation in which the foam is to circulate, may be those previously described and may in addition comprise a condensate trap. This system may also comprise the adjustment and measurement means previously described.
The system for generating a foam and placing a foam in circulation in an installation according to the invention may also comprise a foam flow meter placed downstream from the porous lining such that it is able to measure the quantity of foam generated and to adjust the low pressure in the installation and the inflow rates of the gas and liquid phases into the chamber.
This system may further comprise a foam collector tank placed at the second end of the installation. It may also comprise a pressure sensor, discharge or collection valves for a liquid phase derived from destabilisation of said foam.
According to the invention, the system may also comprise means for collecting a liquid, derived from destabilisation of the foam, in the foam collector tank and means for pumping said liquid as far as the insertion means for adding the liquid phase to the chamber of the foam generation system.
This system may then comprise insulation valves, a system for pumping this liquid from the foam collector tank as far as the preparation and storage tank for the liquid phase used to generate the foam. Said liquid may then, via the insertion means for the liquid phase, be added to the chamber containing the porous lining for example by means of a measuring pump, from the liquid phase preparation and storage tank.
According to the invention, the system may in this case operate either in single flow-through mode or in recycling mode.
When the system of the invention operates in single flow-through mode, the foam is collected and stored in a collector tank which may comprise means for destabilising the foam in order to accelerate its return to the liquid state. Destabilisation may be natural, or accelerated for example using a mechanical system such as those previously described or by chemical means using a destabilising agent such as alcohol for example. The tank may then be emptied by means of a valve either in continuous or periodic manner.
In recovery mode, also called recycling mode, the liquid derived from natural or accelerated destabilisation of the foam, after a first flow through the installation to be decontaminated and/or cleaned for example, is periodically or continuously recovered from the collector tank by means of a recovery pump or recycling pump and is re-injected into the liquid phase preparation and storage tank connected to the measuring pump for the liquid phase.
According to the invention, the recycling operating mode is particularly preferred for industrial application of the proposed decontamination system.
According to the invention, when the recycling mode is used, means for purifying the recovered liquid may be placed downstream from the foam collector tank and upstream from the insertion means for adding the liquid phase to the chamber of the foam generation system, for example from the liquid phase preparation and storage tank.
Other advantages and characteristics of the invention will become clearer on reading the following description given as a non-restrictive illustration, with reference to the appended drawings.